OpenAI Compaction Flaw Exposes Hidden Reasoning in GPT-5.6
Security firm Xint discovered that OpenAI's compaction feature — which compresses long conversations into encrypted context items — could be manipulated to extract a reasoning model's hidden chain-of-thought as plain text. The technique bypassed protections OpenAI had shipped in August 2026 to prevent cross-model reasoning replay attacks, because compaction runs on the same model that generated the reasoning, circumventing the cross-model check. OpenAI confirmed and patched the vulnerability on October 23, 2026, awarding Xint a $6,000 bug bounty, though researchers warn that any server-side step reading decrypted reasoning while following user instructions remains a potential leakage vector.